Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Class A
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) (in dollars)	$ 79,787	$ 58,064	$ 224,060	$ 163,596
Weighted shares (denominator) (in shares)	46,188,941	46,188,949	46,188,522	46,186,109
Per-share amount (in dollars per share)	$ 1.73	$ 1.26	$ 4.85	$ 3.54
Dilutive effect of stock-based awards
Allocated net income (numerator) (in dollars)	$ 0	$ 0	$ 0	$ 0
Weighted shares (denominator) (in shares)	27,697	27,127	24,320	55,541
Assumed conversion of Class B shares
Allocated net income (numerator) (in dollars)	$ 659	$ 479	$ 1,850	$ 1,350
Weighted shares (denominator) (in shares)	6,100,800	6,100,800	6,100,800	6,100,800
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) (in dollars)	$ 80,446	$ 58,543	$ 225,910	$ 164,946
Weighted shares (denominator) (in shares)	52,317,438	52,316,876	52,313,642	52,342,450
Per-share amount (in dollars per share)	$ 1.54	$ 1.12	$ 4.32	$ 3.15
Class B
Reconciliation of the numerators and denominators used in the basic and diluted per-share computations
Ratio for converting shares of Class B common stock into shares of Class A common stock (as a percent)	2400.00%		2400.00%		2400.00%
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) (in dollars)	$ 659	$ 479	$ 1,850	$ 1,350
Weighted shares (denominator) (in shares)	2,542	2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 259	$ 189	$ 728	$ 531
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) (in dollars)	$ 658	$ 479	$ 1,849	$ 1,350
Weighted shares (denominator) (in shares)	2,542	2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 259	$ 189	$ 727	$ 531